U.S. patent number 9,644,606 [Application Number 13/538,161] was granted by the patent office on 2017-05-09 for systems and methods to reduce tower oscillations in a wind turbine.
This patent grant is currently assigned to GENERAL ELECTRIC COMPANY. The grantee listed for this patent is Pranav Agarwal, Arne Koerber, Charudatta Subhash Mehendale. Invention is credited to Pranav Agarwal, Arne Koerber, Charudatta Subhash Mehendale.
United States Patent |
9,644,606 |
Agarwal , et al. |
May 9, 2017 |
Systems and methods to reduce tower oscillations in a wind
turbine
Abstract
Systems and methods to reduce tower oscillations in a wind
turbine are presented. The method includes obtaining a rotor
velocity. Furthermore, the method includes obtaining one or more
parameters associated with a tower of the wind turbine. Further,
the method includes determining a modified rotor velocity based on
the one or more parameters. Moreover, the method includes
determining a first pitch angle based on the modified rotor
velocity. In addition, the method includes pitching one or more
blades of the wind turbine based on the first pitch angle to reduce
the tower oscillations.
Inventors: |
Agarwal; Pranav (Guilderland,
NY), Mehendale; Charudatta Subhash (Niskayuna, NY),
Koerber; Arne (Berlin, DE) |
Applicant: |
Name |
City |
State |
Country |
Type |
Agarwal; Pranav
Mehendale; Charudatta Subhash
Koerber; Arne |
Guilderland
Niskayuna
Berlin |
NY
NY
N/A |
US
US
DE |
|
|
Assignee: |
GENERAL ELECTRIC COMPANY
(Niskayuna, NY)
|
Family
ID: |
48578901 |
Appl.
No.: |
13/538,161 |
Filed: |
June 29, 2012 |
Prior Publication Data
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|
|
Document
Identifier |
Publication Date |
|
US 20140003936 A1 |
Jan 2, 2014 |
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
F03D
7/02 (20130101); F03D 7/0224 (20130101); F03D
7/0276 (20130101); F03D 7/0296 (20130101); F05B
2270/334 (20130101); Y02E 10/72 (20130101) |
Current International
Class: |
F03D
7/02 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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2103915 |
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Sep 2009 |
|
EP |
|
2011000531 |
|
Jan 2011 |
|
WO |
|
2012003985 |
|
Jan 2012 |
|
WO |
|
Other References
Argyris et al., Abstract: Static and Dynamic Investigations on
Different Towers for Wind Turbines, SAO/NASA ADS, Jul. 1979;
2Pages. cited by applicant .
Friedmann, Abstract : Aeroelastic Stability and Response Analysis
of Large Horizontal-Axis Wind Turbines, Journal of Wind Engineering
and Industrial Aerodynamics, vol. 5, Issues 3-4, May 1980; 2 Pages.
cited by applicant .
M.D. Pavel; An Investigation of the Rotor-Tower Instability of the
KEWT Wind Turbine, Delft Aerospace--Memorandum M-879, Project No.
224.740-9854, Nov. 1999; 45 Pages. cited by applicant .
Karl A. Stol et al., Individual Blade Pitch Control for the
Controls Advanced Research Turbine (CART), Journal of Solar Energy
Engineering , vol. 128, Issue 4, Nov. 2006; 3 Pages. cited by
applicant .
M. Geyler et al., Abstract : Robust Multivariable Pitch Control
Design for Load Reduction on Large Wind Turbines, Journal of Solar
Energy Engineering , vol. 130, Issue 3, Aug. 2008; 3 Pages. cited
by applicant .
S. Nourdine et al., Comparison of Wind Turbine LQG Controllers
using Individual Pitch Control to Alleviate Fatigue Loads, 18th
Mediterranean Conference on Control & Automation (MED); Jun.
23-25, 2010; pp. 1591-1596. cited by applicant .
Wind Energy Technology Overview; Downloaded on Jun. 29, 2012 URL :
http://windeis.anl.gov/documents/fpeis/maintext/Vol1/appendices/appendix.-
sub.--d/Vol2AppD.sub.--1.pdf; 44 Pages. cited by applicant .
Leithead et al., "Analysis of Tower/Blade Interaction in the
Cancellation of the Tower Fore-Aft Mode via Control", European Wind
Energy Conference 2004, Nov. 22, 2004, London. cited by
applicant.
|
Primary Examiner: Kershteyn; Igor
Assistant Examiner: Seabe; Justin
Attorney, Agent or Firm: Darling; John P.
Claims
The invention claimed is:
1. A wind turbine, comprising: a rotor comprising one or more rotor
blades; a tower operatively coupled to the rotor; a pitch control
system configured to reduce tower oscillations in the wind turbine,
the pitch control system comprising: a rotor unit configured to
determine a rotor velocity, wherein the rotor unit further
comprises a pitch actuator configured to pitch one or more blades
of the wind turbine based on the first pitch angle; a controller
configured to determine a tower top velocity and a second pitch
angle; and a decoupling unit configured to determine a modified
rotor velocity based on the tower top velocity and the second pitch
angle; wherein the controller is further configured to determine a
first pitch angle based on the modified rotor velocity, and wherein
the decoupling unit further comprises a computing unit configured
to receive the tower top velocity from the rotor unit, receive the
second pitch angle from the controller, determine a first rotor
velocity component based on the tower top velocity, determine a
second rotor velocity component based on the second pitch angle;
and a subtracting unit configured to subtract the first rotor
velocity component and the second rotor velocity component from the
rotor velocity to determine the modified rotor velocity value,
wherein the computing unit is further configured for determining
the first rotor velocity component and the second rotor velocity
component by utilizing a linear model of rotor dynamics, wherein
the linear model is represented by:
.times..delta..times..times..omega..delta..times..times..delta..omega..ti-
mes..delta..times..times..omega..delta..times..times..delta..upsilon..time-
s..delta..times..times..delta..times..times..delta..times..times..theta..t-
imes..delta..theta. ##EQU00007## or approximations thereof, where
J.sub.r is the moment of inertia of a rotor, .delta.{circumflex
over (.omega.)}.sub.rc is a combination of the first rotor velocity
component and the second rotor velocity component,
.delta.{circumflex over ({dot over (.omega.)})}.sub.rc is rate of
change of the combination of the first rotor velocity component and
the second rotor velocity component, .delta.{dot over (X)}.sub.fa
is the tower to velocity, and .delta..theta..sub.twr is the second
pitch angle.
Description
BACKGROUND
Embodiments of the present disclosure relate to wind turbines, and
more particularly to reducing tower oscillations in wind
turbines.
Modern wind turbines operate in a wide range of wind conditions.
These wind conditions can be broadly divided into two
categories--below rated speeds and above rated speeds. To produce
power in these wind conditions, wind turbines may include
sophisticated control systems such as pitch controllers and torque
controllers. These controllers account for changes in the wind
conditions and accompanying changes in wind turbine dynamics. For
example, pitch controllers generally vary the pitch angle of rotor
blades to account for the changes in wind conditions and turbine
dynamics. During below rated wind speeds, wind power may be lower
than the rated power output of the wind turbine. In this situation,
the pitch controller may attempt to maximize the power output by
pitching the rotor blades substantially perpendicular to the wind
direction. Alternatively, during above rated wind speeds, wind
power may be greater than the rated power output of the wind
turbine. Therefore, in this case, the pitch controller may restrain
wind energy conversion by pitching the rotor blades such that only
a part of the wind energy impinges on the rotor blades. By
controlling the pitch angle, the pitch controller thus controls the
velocity of the rotor blades and in turn the energy generated by
the wind turbine.
Along with maintaining rotor velocity, pitch controllers may also
be employed to reduce tower oscillations. Tower oscillations or
vibrations occur due to various disturbances, such as turbulence,
inefficient damping, or transition between the two wind conditions.
Moreover, the tower may vibrate along any degree of freedom. For
example, the tower may vibrate in a fore-aft direction (commonly
referred to as tower nodding), in a side-to-side direction
(commonly referred to as tower naying), or along its longitudinal
axis (commonly referred to as torsional vibration).
Tower nodding is usually caused by aerodynamic thrust and rotation
of the rotor blades. Every time a rotor blade passes in front of
the tower, the thrust of the wind impinging on the tower decreases.
Such continuous variation in wind force may induce oscillations in
the tower. Moreover, if the rotor velocity is such that a rotor
blade passes over the tower each time the tower is in one of its
extreme positions (forward or backward), the tower oscillations may
be amplified. Typically, the oscillations in the fore-aft direction
are automatically minimized due to aerodynamic damping. Aerodynamic
damping relies on the fact that the top of the tower constantly
oscillates in the fore-aft direction. When the top of the tower
moves upwind (or forward), the rotor thrust is increased. This
increase in rotor thrust pushes the tower back downwind. The
downwind push in turn aids in dampening the tower oscillations.
Similarly, when the top of the tower moves downwind, the rotor
thrust may be decreased. This decrease in rotor thrust pushes the
tower back upwind. The upwind push also aids in dampening the tower
oscillations.
Although aerodynamic damping aids in reducing oscillations
considerably, if the rotor velocity is synchronized with the tower
oscillations, the results may be detrimental for wind turbine
components. In such instances, the tower may oscillate at a high
rate causing mechanical strain and possible damage to the tower.
Moreover, such synchronization may amplify the rotor velocity at
tower resonance frequency, thereby potentially damaging generators
and/or drivetrains connected to the rotor blades. As the
amplification of tower oscillations is dependent on the rotor
velocity, pitching the rotor to adjust its velocity may prevent
amplification of the tower oscillations. Accordingly, by pitching
the rotor blades, the pitch controller may control the rotor
velocity and prevent amplification of the tower oscillations.
Typically, the pitch controller utilizes two separate control loops
for the two functions--controlling the rotor velocity and reducing
the tower oscillations. A rotor velocity control loop is employed
to determine a pitch angle to control rotor velocity and a
tower-damping control loop is used to compute a pitch angle to
reduce tower oscillations. Often, these feedback loops operate
relatively independently of each other. For example, the rotor
velocity control loop may determine the pitch angle based on rotor
velocity, wind speed, and current pitch angle. The tower-damping
control loop, on the other hand, may determine the pitch angle
based on tower deflection, tower top velocity, tower top
acceleration, current pitch angle, and wind speed. Because of this
independence, the currently available rotor velocity control loops
may compute a pitch angle to maintain rotor speed that may
disadvantageously induce tower oscillations instead of reducing
them. Moreover, these rotor velocity control loops may cause energy
amplification in the rotor near tower resonance frequencies. Such
amplification may increase oscillations in the tower and increase
the fatigue load placed on the wind turbine. Over time, such
fatigue loads may reduce the life of wind turbine parts and
increase the expenses associated with wind turbines.
BRIEF DESCRIPTION OF THE INVENTION
In accordance with aspects of the present disclosure, a method for
reducing tower oscillations in a wind turbine is presented. The
method includes obtaining a rotor velocity. Furthermore, the method
includes obtaining one or more parameters associated with a tower
of the wind turbine. Further, the method includes determining a
modified rotor velocity based on the one or more parameters.
Moreover, the method includes determining a first pitch angle based
on the modified rotor velocity. In addition, the method includes
pitching one or more blades of the wind turbine based on the first
pitch angle to reduce the tower oscillations.
In accordance with another aspect of the present disclosure, a
pitch control system is presented. The pitch control system
includes a tower unit configured to determine one or more
parameters associated with a tower of a wind turbine. Further, the
pitch control system includes a decoupling unit configured to
determine a modified rotor velocity based on the one or more
parameters. Additionally, the pitch control system includes a
controller configured to determine a first pitch angle based on the
modified rotor velocity.
In accordance with yet another aspect of the present disclosure, a
wind turbine is presented. The wind turbine includes a rotor having
one or more rotor blades and a tower operatively coupled to the
rotor. Further, the wind turbine includes a pitch control system
for reducing tower oscillations in the wind turbine. The pitch
control system includes a rotor unit configured to determine a
rotor velocity, a tower unit configured to determine at least one
of a tower top velocity and a second pitch angle, a decoupling unit
configured to determine a modified rotor velocity based on at least
one of the tower top velocity and the second pitch angle, and a
controller configured to determine a first pitch angle based on the
modified rotor velocity.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other features, aspects, and advantages of the present
disclosure will be better understood when the following detailed
description is read with reference to the accompanying drawings in
which like characters represent like parts throughout the drawings,
wherein:
FIG. 1 is a diagrammatical representation of forces and motions
experienced by a wind turbine;
FIG. 2 is a diagrammatical representation of an exemplary pitch
control system, according to aspects of the present disclosure;
FIG. 3 is a graph illustrating energy amplification in rotor
velocity of a conventional wind turbine at different wind
speeds;
FIG. 4 is a graph illustrating energy amplification in rotor
velocity of a wind turbine employing the exemplary pitch control
system of FIG. 2 at different wind speeds, according to aspects of
the present disclosure;
FIG. 5 is a diagrammatical representation of another exemplary
pitch control system, according to aspects of the present
disclosure;
FIG. 6 is a graph illustrating energy amplification in rotor
velocity of a conventional wind turbine with a tower-damping unit
at different wind speeds;
FIG. 7 is a graph illustrating energy amplification in rotor
velocity of a wind turbine employing the exemplary pitch control
system of FIG. 5 at different wind speeds, according to aspects of
the present disclosure;
FIG. 8 is a flowchart illustrating an exemplary method for reducing
tower oscillations in a wind turbine using the pitch control system
of FIG. 2, according to aspects of the present disclosure; and
FIG. 9 is a flowchart illustrating an exemplary method for reducing
tower oscillations in a wind turbine using the pitch control system
of FIG. 5, according to aspects of the present disclosure.
DETAILED DESCRIPTION
The following terms, used throughout this disclosure, may be
defined as follows:
Tower dynamics--refers to the mechanics concerned with the motion
of a wind turbine tower under the action of various forces such as
wind and rotor movement.
Rotor Dynamics--refers to the mechanics concerned with the motion
of the rotor under the action of various forces such as wind, tower
movement, and inertia.
Fore-aft oscillations--refers to tower oscillations in a direction
parallel to the wind direction.
Tower top velocity--refers to the velocity of the tower
oscillations experienced at the top end of a wind turbine
tower.
Tower top acceleration--refers to the acceleration of the tower
oscillations experienced at the top of the wind turbine tower.
Tower deflection--refers to the change in position of the top of
the wind turbine tower with respect to a reference position.
Tower resonance--refers to the tendency of a wind turbine to
oscillate with maximum amplitude at tower resonant frequencies.
First mode resonance frequency--refers to the resonant frequency of
a first structural mode of the wind turbine tower where the mode
dynamics are characterized by a second order spring-mass-damper
system.
Embodiments of the present disclosure are related to an exemplary
system and method for reducing tower oscillations in a wind
turbine. More particularly, the present disclosure relates to an
exemplary rotor velocity control loop that uses a pitch control
system as an actuator. Moreover, the rotor velocity control loop
determines a pitch angle that reduces tower oscillations. To this
end, the rotor velocity control loop includes a decoupling unit
that addresses the interdependence between rotor dynamics and tower
dynamics using model based methods to reduce oscillations induced
in the tower fore-aft direction at above rated speeds.
Moreover, embodiments of the present disclosure are described with
reference to a land-based three-blade wind turbine. It will be
understood, however, that such a reference is merely exemplary and
that the systems and methods described here may just as easily be
implemented in floating wind turbines, offshore wind turbines,
2-blade wind turbines, or 4-blade wind turbines without departing
from the scope of the present disclosure.
FIG. 1 is a diagrammatical representation that illustrates forces
and motions experienced by a wind turbine 100. The wind turbine 100
includes a tower 102, a rotor 104, one or more rotor blades 106,
and a nacelle 108. The tower 102 may be coupled to the ground, to
the ocean floor, or to a floating foundation using any known
securing means, such as bolting, cementing, welding, and so on.
Further, in FIG. 1 reference numeral 110 is generally
representative of wind. The wind 110 may have a mean speed
(.upsilon.). As the wind 110 blows in the indicated direction, an
aerodynamic torque (M.sub.z) is placed on the rotor blades 106
causing the rotor blades 106 to rotate in a direction that is
substantially perpendicular to the wind direction. This motion of
the rotor blades 106 is represented in FIG. 1 by an angular rotor
velocity (.omega..sub.r) of the rotating blades 106. Further, the
nacelle 108 may include a gearbox (not shown) and a generator (not
shown). The gearbox may increase the speed of the rotor blades 106
and the generator may convert the rotation of the rotor blades 106
into electricity, thus converting the energy of the wind 110 into
electricity. Alternatively, the nacelle 108 may include a direct
drivetrain (not shown). In such cases, inclusion of the gearbox may
be circumvented.
Moreover, due to an aerodynamic thrust (F.sub.z) of the wind 110
and the rotation of the rotor blades 106, the tower 102 may
oscillate in a fore-aft direction. Reference numeral 114 is
generally representative of the fore-aft oscillations. It will be
understood that in addition to the fore-aft oscillations 114, the
tower 102 may also experience other oscillations. Example
oscillations include side-to-side oscillations, torsional
oscillations, twisting oscillations, and the like. These
oscillations are not illustrated in FIG. 1.
The wind turbine 100 may employ a sensing device to detect the
fore-aft oscillations 114. For example, an oscillation velocity
detector (not shown) or an oscillation deflection detector (not
shown) may be employed. Alternatively, an accelerometer 112 may be
employed in the wind turbine 100 to detect the acceleration of the
fore-aft oscillations 114. In some embodiments, the accelerometer
112 may be disposed within the nacelle 108 or at the top of the
tower 102. In other instances, the accelerometer 112 may be
positioned at the center of the tower 102.
Furthermore, to reduce the fore-aft tower oscillations 114 and to
control the rotor velocity, the wind turbine 100 may include an
exemplary pitch control system 116 that may include a rotor
velocity control loop (not shown). In some embodiments, the pitch
control system 116 may also include a tower-damping control loop
(not shown). Depending on the mean or effective speed of the
incoming wind 110, the exemplary pitch control system 116 may be
configured to determine the pitch angle of the rotor blades 106 to
maximize output power (within the rated limits) and/or minimize
tower oscillations. As noted previously, some of the previously
known pitch controllers may tend to increase tower oscillations,
instead of decreasing them. This increase in tower oscillations may
be because conventional pitch controllers fail to account for the
interdependence between rotor dynamics and tower dynamics.
Tower dynamics for the wind turbine 100, in one example, may be
represented by a second order linear equation: {umlaut over
(X)}.sub.fa+2.xi..sub.fa.omega..sub.fa{dot over
(X)}.sub.fa+X.sub.fa=KF.sub.z(.omega..sub.r,.theta.,.upsilon..sub.e)
(1) where, {umlaut over (X)}.sub.fa is the tower top acceleration,
.xi..sub.fa is the velocity-damping constant of the tower 102,
.omega..sub.fa is the first mode tower resonant frequency, {dot
over (X)}.sub.fa is the tower top velocity, and X.sub.fa is the
tower deflection. Further, K is an inverse of a generalized mass
for the first mode, F.sub.z is the aerodynamic thrust,
.omega..sub.r is the angular velocity, .theta. is the pitch angle,
and .upsilon..sub.e is the effective wind speed.
The effective wind speed (.upsilon..sub.e) refers to the effective
speed of the wind at the hub height of the wind turbine 100.
Because the wind 110 is distributed spatially and temporally, the
wind speed varies significantly at different points over the area
swept by the rotor blades 106, and therefore different portions of
the wind turbine 100 may experience different wind speeds. The
effective wind speed (.upsilon..sub.e) is representative of the
difference between the mean wind speed (.upsilon.) and the tower
top velocity ({dot over (X)}.sub.fa) as depicted in equation (2):
.upsilon..sub.e=.upsilon.-{dot over (X)}.sub.fa (2)
The left-hand side of equation (1) indicates that the motion of the
tower 102 may be dependent on the tower top acceleration ({umlaut
over (X)}.sub.fa), tower top velocity ({dot over (X)}.sub.fa),
tower deflection (X.sub.fa), resonant frequency (.omega..sub.fa),
and velocity-damping constant (.xi..sub.fa). In addition, the
right-hand side of equation (1) illustrates that the aerodynamic
thrust (F.sub.z) experienced by the tower 102 may be a function of
the angular velocity (.omega..sub.r), the pitch angle (.theta.),
and the effective wind speed (.upsilon..sub.e). Further, the
aerodynamic thrust (F.sub.e) may be a function of the mean wind
speed (.upsilon.) and the tower top velocity ({dot over
(X)}.sub.fa) as these parameters affect the effective wind speed
(.upsilon..sub.e).
Moreover, rotor dynamics for the wind turbine 100 may also be
represented by a first order linear equation: J.sub.r{dot over
(.omega.)}.sub.r=M.sub.z(.omega..sub.r,.theta.,.upsilon..sub.e)-NT.sub.g
(3) where, J.sub.r is a moment of inertia of the rotor 104, {dot
over (.omega.)}.sub.r is the rate of change in the angular velocity
of the rotor, N is gearbox ratio, and T.sub.g is the generator
reaction torque.
It will be noted that both the rotor dynamics and the tower
dynamics depend on the effective wind speed (.upsilon..sub.e).
Further, it will be noted that the effective wind speed
(.upsilon..sub.e) is a function of the tower top velocity ({dot
over (X)}.sub.fa). Therefore, it is evident from equations (1) and
(3) that the tower dynamics and the rotor dynamics are dependent on
each other. In fact, these dynamics are related to each other
because of the tower top velocity ({dot over (X)}.sub.fa), rotor
velocity (.omega..sub.r), and pitch angle (.theta.).
Conventional pitch controllers typically assume that the rotor
dynamics and the tower dynamics are independent. Consequently,
these pitch controllers generally ignore the tower top velocity
while computing the pitch angle for controlling the rotor velocity
and/or damping the tower oscillations. Moreover, because of this
exclusion, conventional pitch controllers may cause instability in
the rotor dynamics and energy amplification in the rotor velocity
at frequencies close to the tower resonance. In one embodiment, the
exemplary pitch control system 116 may be configured to employ the
tower top velocity in the computation of the pitch angle. More
particularly, the exemplary pitch control system 116 may be
configured to deduct the effects of the tower top velocity from the
rotor velocity. By including the tower top velocity and
compensating for this value in the computation of the pitch angle,
the exemplary pitch control system 116 may advantageously decouple
the rotor dynamics and the tower dynamics.
FIG. 2 illustrates an exemplary embodiment 200 of the pitch control
system 116 of FIG. 1, according to aspects of the present
disclosure. The pitch control system 200 of FIG. 2 includes a rotor
velocity control loop. Further, the pitch control system 200 may
include a rotor unit 202, a tower unit 204, and a controller 206.
Moreover, the pitch control system 200 may also include a
decoupling unit 208. In one embodiment, the controller 206 may be
disposed in a feedback loop of the rotor unit 202 and the
decoupling unit 208 may be disposed at an output of the rotor unit
202 and the tower unit 204.
The rotor unit 202 may be configured to determine a rotor velocity
(.omega..sub.r). In one embodiment, the rotor unit 202 may be
configured to determine the rotor velocity (.omega..sub.r) by
directly measuring the angular speed of the rotor 104 (see FIG. 1)
using a sensing device such as a speedometer or an angular velocity
meter. Alternatively, the rotor unit 202 may be configured to
determine the rotor velocity (.omega..sub.r) by determining a power
output of the wind turbine 100 (see FIG. 1) or the rotation speed
of a generator. It may be noted that these values are proportional
to the rotor velocity. Accordingly, determination of any of these
parameters may aid the rotor unit 202 in determining the rotor
velocity. It will be understood that various models and measurement
means may be employed to determine the rotor velocity and any of
these models or means may be employed to determine the rotor
velocity without departing from the scope of the present
disclosure.
The tower unit 204 may be configured to determine one or more
parameters associated with the tower 102. These parameters may be
representative of the tower dynamics. For instance, in one
embodiment of the pitch control system 200, the tower unit 204 may
be configured to determine the tower top velocity ({dot over
(X)}.sub.fa). The tower top velocity ({dot over (X)}.sub.fa) may be
estimated using the tower top acceleration ({umlaut over
(X)}.sub.fa). As previously noted, the accelerometer 112 (see FIG.
1) may be employed to sense the tower top acceleration and
communicate this information to the tower unit 204. The tower unit
204 may be configured to perform any known computation to determine
the tower top velocity ({dot over (X)}.sub.fa). For instance, the
tower unit 204 may be configured to determine the tower top
velocity ({dot over (X)}.sub.fa) by performing an integration
operation on the tower top acceleration ({umlaut over (X)}.sub.fa).
Alternatively, the tower unit 204 may determine the tower top
velocity ({dot over (X)}.sub.fa) from the tower acceleration
({umlaut over (X)}.sub.fa) using a model based estimator such as a
Kalman Filter.
In other embodiments, the tower top velocity ({dot over
(X)}.sub.fa) may be estimated by a deflection sensor that detects a
degree of deflection of the tower 102 about a determined rest
position. By measuring the deflection at various instances of time,
the tower top velocity ({dot over (X)}.sub.fa) may be determined.
In another embodiment, the tower unit 204 may be configured to
perform a differentiation operation on the tower deflection to
determine the tower top velocity ({dot over (X)}.sub.fa). In yet
another embodiment, the tower top velocity ({dot over (X)}.sub.fa)
may be directly sensed by a velocity sensor. It will be understood
that the tower unit 204 may perform various other functions and
operations without departing from the scope of the present
disclosure. For example, the tower unit 204 may maintain and
continuously update a model of the tower dynamics.
In accordance with aspects of the present disclosure, the
decoupling unit 208 may be configured to determine a modified rotor
velocity based on parameters of the tower 102. To this end, the
decoupling unit 208 may include a computing unit 210 and a
subtracting unit 212. The computing unit 210 may be configured to
receive the parameters associated with the tower 102. By way of
example, the computing unit 210 may be configured to receive the
tower top velocity from the tower unit 204. Furthermore, the
computing unit 210 may be configured to determine a rotor velocity
component based on the tower top velocity (hereinafter referred to
as the "first rotor velocity component"). The first rotor velocity
component may be representative of the effect of the tower top
velocity on the rotor velocity. To determine the first rotor
velocity component, the computing unit 210 may utilize a linear
model of the rotor dynamics. The rotor dynamics may be represented
by the following first order linear equation:
.times..delta..times..times..omega..delta..times..times..delta..omega..ti-
mes..delta..omega..delta..times..times..delta..times..times..theta..times.-
.delta..theta..delta..times..times..delta..upsilon..times..delta..times..t-
imes..upsilon..delta..times..times. ##EQU00001## or approximations
thereof, where
.delta..times..times..delta..omega. ##EQU00002## is the partial
derivative of the aerodynamic torque with respect to the rotor
velocity,
.delta..times..times..delta..theta. ##EQU00003## is the partial
derivative or me aerodynamic torque with respect to the pitch
angle, and
.delta..times..times..delta..upsilon. ##EQU00004## is the partial
derivative of the aerodynamic torque with respect to the mean wind
velocity.
Further, a linear model of the rotor dynamics may be represented by
the following equation:
.times..delta..times..times..omega..delta..times..times..delta..omega..ti-
mes..delta..times..times..omega..delta..times..times..delta..times..times.-
.upsilon..times..delta..times..times. ##EQU00005## or
approximations thereof, where .delta.{circumflex over ({dot over
(.omega.)})}.sub.rf is the rate of change of the first rotor
velocity component and .delta.{circumflex over (.omega.)}.sub.rf is
the first rotor velocity component.
It may be noted that all the variables in equation (5), with the
exception of the first rotor velocity component, may be detected
and/or stored by the rotor unit 202 and/or the tower unit 204. The
values of these variables may be communicated to the computing unit
210. The computing unit 210 may be configured to compute the first
rotor velocity component based on the values of these
variables.
Moreover, in one example, the computing unit 210 may be implemented
as one or more digital filters. In another example, the computing
unit 210 may be implemented as a general-purpose computing device.
The general-purpose computing device may be selectively activated
or reconfigured by a decoupling means/unit. For example, the
computing device may store the rotor dynamics and the linearized
model of the rotor dynamics in a non-transitory computer readable
storage medium, such as, but not limited to, any type of disk,
memory, magnetic card, optical card, or any type of media suitable
for storing electronic instructions. Further, the computing device
may store instructions or programs configured to compute the first
rotor velocity component.
As described previously, the decoupling unit 208 may further
include the subtracting unit 212 that may be configured to receive
the rotor velocity (.omega..sub.r) from the rotor unit 202 and the
first rotor velocity component (.delta.{circumflex over
(.omega.)}.sub.rf) from the computing unit 210. Moreover, the
subtracting unit 212 may be configured to subtract the first rotor
velocity component (.delta.{circumflex over (.omega.)}.sub.rf) from
the rotor velocity (.omega..sub.r) to obtain a modified rotor
velocity. The modified rotor velocity may be representative of the
rotor velocity that is devoid of the effects of the tower top
velocity.
The controller 206 may be configured to receive the modified rotor
velocity, process this value, and generate a pitch angle value
(.delta..theta.) corresponding to the modified rotor velocity
(hereinafter referred to as a "first pitch angle"). To process this
value, in one embodiment, the controller 206 may include a lookup
table (LUT) that includes previously computed pitch angle values
corresponding to various rotor velocities. The modified rotor
velocity may be compared with the stored rotor velocities to
determine a corresponding first pitch angle. Alternatively, the
controller 206 may include a threshold rotor velocity. In this
case, the modified rotor velocity may be compared with a threshold
rotor velocity. Further, the controller 206 may be configured to
generate an error signal indicative of any deviation of the
modified rotor velocity from the threshold rotor velocity. The
controller 206 may further include a LUT to store pitch angle
values corresponding to various error values. By performing a
lookup in such a table, the controller 206 may be configured to
determine an appropriate first pitch angle. In other embodiments of
the controller 206, the first pitch angle may be computed in real
time by utilizing one or more known wind turbine models that may be
stored in an associated LUT.
In some instances, independent pitching of the rotor blades 106 may
further reduce the oscillations and increase the efficiency of the
wind turbine 100. In such instances, the controller 206 may be
configured to independently determine first pitch angles for each
rotor blade 106. Techniques for such computations may include
receiving modified rotor velocities corresponding to each rotor
blade 106 separately or receiving a single modified rotor velocity.
In case of individual modified rotor velocities, the controller 206
may be configured to perform a simple lookup in the LUT to
determine the individual first pitch angles. Otherwise, the
controller 206 may be configured to utilize one or more wind
turbine models to determine the individual first pitch angles. For
example, during the turbine design phase, various calculations may
be carried out to determine a model for defining the rotor velocity
attained at various individual pitch angles and wind speeds. The
results of such computations may be stored in the controller 206.
Subsequently, during operation, the controller 206 may be
configured to perform a lookup to determine the individual first
pitch angles that may be utilized to attain the modified rotor
velocity. Alternatively, the controller 206 may be configured to
supply the modified rotor velocity, previous pitch angles, and
current wind speed to the model to determine the individual first
pitch angles. It will be understood that various pitch angle
controllers are currently employed in wind turbines and that any of
these pitch controllers may be utilized to implement the controller
206 without departing from the scope of the present disclosure. The
controller 206 may be any of the controllers known in the art, such
as a proportional controller, a proportional integral controller, a
proportional-integral-derivative controller, a linear-quadratic
regulator, or a linear-quadratic Gaussian regulator without
departing from the scope of the present disclosure.
In some embodiments, the rotor unit 202 may include a pitch
actuator 214 for pitching the rotor blades 106 based on the first
pitch angle determined by the controller 206. As described
previously, the controller 206 may be configured to generate and
transmit substantially similar first pitch angles for the blades in
the wind turbine 100 to the pitch actuator 214. Alternatively, the
controller 206 may transmit independent first pitch angles to the
pitch actuator 214. The pitch actuator 214, in turn, may include
any actuation mechanism to adjust the pitch angle of the rotor
blades 106. For example, the pitch actuator 214 may be a hydraulic
system that receives pitch angle values in the form of voltage
signals and pitches the rotor blades 106 by actuating a pitch
cylinder (not shown) at a variable rate. Alternatively, the pitch
actuator 214 may be an electrical, electronic, or
electro-mechanical system without departing from the scope of the
present disclosure.
It may be noted that FIG. 2 illustrates the decoupling unit 208 and
the controller 206 as separate hardware units. However, it will be
understood that in some embodiments, the controller 206 may be
designed as a multi-input and multi-output (MIMO) controller that
includes the functionality of the decoupling unit 208 and/or the
rotor and tower units 202 and 204. In embodiments where the
controller 206 includes the decoupling unit 208, the tower top
velocity and the rotor velocity may be directly provided to the
controller 206. The controller 206, in turn, may include the
computing unit 210 and the subtracting unit 212 to compute the
first rotor velocity component and subtract this value from the
detected rotor velocity, respectively. Based on the subtraction,
the controller 206 may determine the modified rotor velocity.
FIGS. 3 and 4 are graphs 300, 400 schematically illustrating
simulated energy amplification in rotor velocity of a wind turbine,
at various wind speeds. Further, these graphs 300, 400 illustrate
the energy amplification of the rotor velocity using pitch angle as
an actuator. More particularly, graph 300 illustrates the effect of
a conventional pitch control system (without the decoupling unit)
on the energy amplification in the rotor velocity of a conventional
wind turbine at different wind speeds and frequencies. Graph 400
illustrates the effect of the exemplary pitch control system 200 of
FIG. 2 on the energy amplification in the rotor velocity of the
wind turbine 100 at different wind speeds and frequencies.
Graph 300 illustrates that there is significant energy
amplification at the tower resonance frequency (generally indicated
by reference numeral 302). In essence, such amplification occurs
because conventional pitch controllers do not account for the tower
top velocity while determining the pitch angle to control the rotor
velocity.
To circumvent the shortcomings of the conventional pitch
controllers, the exemplary decoupling unit 208 of FIG. 2 may be
configured to prevent energy amplification and reduce fore-aft
oscillations 114 (see FIG. 1) at tower resonance frequencies. In
particular, the decoupling unit 208 may be configured to determine
a rotor velocity component that results from the tower
oscillations. Additionally, the decoupling unit 208 may be
configured to deduct this component from the rotor velocity.
Consequently, the effects of the tower oscillations on the rotor
velocity may be substantially minimized. Accordingly, wind speed
and pitch angle may be the only factors that affect the modified
rotor velocity. Graph 400 illustrates this statement. It will be
appreciated that the energy amplification of FIG. 3 is not present
in FIG. 4. Therefore, introduction of the exemplary decoupling unit
208 in the pitch control system 200 aids in minimizing energy
amplification and subsequent tower oscillations.
FIG. 5 is a diagrammatical representation of another exemplary
embodiment 500 of the pitch control system 116 of FIG. 1. In this
embodiment, the pitch control system 500 includes a rotor velocity
control loop and a tower-damping control loop. Accordingly, the
pitch control system 500 includes a rotor unit 502, a tower unit
504, and a controller 506. These units function substantially
similar to the similarly named units described with reference to
FIG. 2. Furthermore, the pitch control system 500 may include a
tower-damping unit 508, a decoupling unit 510, and an adder 512.
The tower-damping unit 508 may be coupled between an output of the
tower unit 504 and an input of the rotor unit 502. Also, the
decoupling unit 510 may be coupled at an output of the rotor unit
502, tower unit 504, and the tower-damping unit 508. Further, the
adder 512 may be coupled between an output of the controller 506
and the tower-damping unit 508, and an input of the rotor unit
502.
The tower-damping unit 508 may be configured to reduce the
oscillations in the tower 102 of FIG. 1. As previously noted with
reference to FIG. 1, these oscillations are typically caused by
disturbances in the wind 110, operation of the rotor blades 106, or
any other such factors. During operation of the wind turbine 100, a
lift and a drag act on the rotor blades 106. The drag acts as a
thrust in the front-rear direction of the tower 102, thereby
inducing fore-aft oscillations 114. Moreover, the magnitude of the
thrust varies depending on the wind speed and the pitch angle.
Accordingly, by controlling the pitch angle, the thrust in the
front-rear direction may be adjusted, which in turn regulates the
fore-aft oscillations 114.
With continuing reference to FIG. 5, in accordance with some
aspects of the present disclosure, the tower-damping unit 508 may
be configured to calculate a pitch angle for generating a desired
thrust on the rotor blades 106. In one example, the desired thrust
may be representative of the thrust that may be applied on the
rotor blades 106 to substantially minimize or cancel the
oscillations of the tower 102. Further, the tower-damping unit 508
may determine the pitch angle based on the detected tower top
acceleration. Subsequently, the adder 512 may add the pitch angle
for damping (hereinafter referred to as the "second pitch angle")
with the first pitch angle to generate a combined pitch angle. The
combined pitch angle may be employed to pitch the rotor blades
106.
Despite reducing oscillations caused by the aerodynamic thrust
(F.sub.z), conventional tower dampers may introduce energy
amplification in the rotor at tower resonance. This amplification
may occur because conventional pitch controllers ignore the effects
of the second pitch angle on the first pitch angle while computing
the first pitch angle. In accordance with aspects of the present
disclosure, embodiments of the pitch control system 500 account for
the effects of the second pitch angle on the first pitch angle. In
particular, the pitch control system 500 may be configured to
deduct these effects along with the effects of the tower top
velocity from the rotor velocity to determine a modified rotor
velocity. By minimizing and/or removing the effects of the second
pitch angle and the tower top velocity from the rotor velocity,
embodiments of the pitch control system 500 aid in reducing or
eliminating the possibility of energy amplification at tower
resonance frequencies in the rotor 104 (see FIG. 1).
To obtain the modified rotor velocity, the decoupling unit 510 may
be configured to determine components of rotor velocity based on
one or more parameters associated with the tower 102, such as the
tower top velocity and the second pitch angle. More particularly,
the decoupling unit 510 may be configured to determine a component
of rotor velocity due to the second pitch angle (hereinafter
referred to as the "second rotor velocity component") in addition
to the first rotor velocity component. Accordingly, the decoupling
unit 510 may be configured to receive the tower top velocity from
the tower unit 504 and the second pitch angle from the
tower-damping unit 508. In one embodiment, the decoupling unit 510
may include a computing unit 514 and a subtracting unit 516. The
computing unit 514 may be configured to determine the first rotor
velocity component and the second rotor velocity component using a
linearized model of the rotor dynamics, in one example.
Accordingly, in this embodiment, the linearized model may include
the second pitch angle in addition to the tower top velocity. The
linearized model of the rotor dynamics may be represented by the
following equation:
.times..delta..times..times..omega..delta..times..times..delta..omega..ti-
mes..delta..times..times..omega..delta..times..times..delta..upsilon..time-
s..delta..times..times..delta..times..times..delta..times..times..theta..t-
imes..delta..theta. ##EQU00006## or approximations thereof, where,
.delta.{circumflex over (.omega.)}.sub.rc is a combination of the
first rotor velocity component and the second rotor velocity
component, .delta.{circumflex over ({dot over (.omega.)})}.sub.rc
is the rate of change of the combination of the first and second
rotor velocity components, and .delta..theta..sub.twr is the second
pitch angle.
The computing unit 514 may be configured to retrieve the second
pitch angle and the tower top velocity from the tower-damping unit
508 and the tower unit 504, respectively. Based on these values,
the computing unit 514 may be configured to determine a combination
of the first and second components of the rotor velocity due to
tower oscillations and tower damping. To determine the modified
rotor velocity, the subtracting unit 516 may be configured to
deduct the combination of the first and second rotor velocity
components from the rotor velocity.
According to one embodiment, the decoupling unit 510 may be
implemented as one or more digital filters or a computing
device--one for determining the first rotor velocity component and
the other for determining the second rotor velocity component.
Alternatively, the decoupling unit 510 may be implemented as a
single digital filter or computing device that may be configured to
determine both the first and second rotor velocity components
simultaneously.
The other units, such as the controller 506 and the rotor unit 502,
may function in a manner that is substantially similar to the
operation of their counterparts as described with reference to FIG.
2. For instance, the rotor unit 502 may be configured to
communicate the detected rotor velocity to the subtracting unit
516. Similarly, the controller 506 may be configured to determine
the first pitch angle and provide this value to the adder 512.
Furthermore, the adder 512, in turn, may be configured to receive
the first pitch angle and the second pitch angle and combine these
two values to determine a combined pitch angle. This combined pitch
angle may be communicated to a pitch actuator 518. Further, the
pitch actuator 518 may be configured to pitch the rotor blades
according to the communicated pitch angle.
FIGS. 6 and 7 are graphs 600, 700 schematically illustrating
simulated energy amplification in rotor velocity of a wind turbine.
Further, these graphs 600, 700 illustrate energy amplification
using pitch angle as an actuator. More particularly, FIG. 6
illustrates the effect of a conventional pitch controller (with a
tower damping loop) on the energy amplification in the rotor
velocity at different wind speeds and frequencies. FIG. 7
illustrates the effect of the exemplary pitch control system 500 of
FIG. 5 on the energy amplification in the rotor velocity at
different wind speeds and frequencies.
Graph 600 illustrates that there is significant energy
amplification at the tower resonance frequency, generally
represented by reference numeral 602. It may be noted that the
energy amplification in this case is not as severe as in FIG. 3 due
to the inclusion of a tower-damping loop in this conventional pitch
controller. Graph 700 illustrates that the peak of the energy
amplification indicated in FIG. 6 is significantly reduced by
implementing the decoupling unit 510 of the pitch control system
500. Therefore, by introducing the decoupling unit 510, energy
amplification at tower resonance frequencies may be prevented and
excessive tower oscillations because of amplitude amplification may
be circumvented.
FIG. 8 is a flow chart 800 that illustrates an exemplary method for
reducing oscillations in a wind turbine. The method will be
described with reference to FIGS. 1-2. The method begins at step
802 where a rotor velocity of a wind turbine, such as the wind
turbine 100, is determined. In one embodiment, the rotor unit 202
may be configured to determine the rotor velocity by directly
measuring the rotor velocity using a sensor, such as an anemometer,
a speedometer, a rotational velocity meter, and so on.
Alternatively, the rotor unit 202 may be configured to determine
the rotor velocity by measuring an output power or generator speed
of the wind turbine 100. In this case, the rotor velocity may be
estimated as the velocity that generates the corresponding output
power or generator speed.
Subsequently, at step 804, one or more parameters associated with a
tower, such as the tower 102, may be determined. More particularly,
a tower top velocity may be determined. In one embodiment, the
tower unit 204 may be configured to determine the tower top
velocity based on a tower top acceleration. The accelerometer 112
coupled to the wind turbine 100 may be employed to determine the
acceleration of the tower deflections. Based on this detected
value, the tower unit 204 may compute the tower top velocity. By
way of example, the tower unit 204 may perform an integration
operation on the tower top acceleration to determine the tower top
velocity. Alternatively, the tower velocity may be determined from
available measurements such as tower acceleration using a
model-based estimator such as a Kalman filter. In other
embodiments, a velocity sensor or a deflection sensor may be
installed on the wind turbine 100 to measure the tower top velocity
or the tower deflection, respectively. In case the tower deflection
is detected, the tower unit 204 may be configured to perform a
differentiation operation on the tower deflection to determine the
tower top velocity. Furthermore, one or more of the sensors may be
coupled to the tower unit 204 such that the measured parameter
value may be directly provided to the tower unit 204.
Furthermore, at steps 806 and 808, a modified rotor velocity may be
computed. To this end, a first rotor velocity component may be
computed, as indicated by step 806. The computing unit 210 may be
configured to utilize a linearized model of the rotor dynamics as
represented by equation (5) to determine the modified rotor
velocity. By substituting the tower top velocity and other variable
values in equation (5), the computing unit 210 may determine the
first rotor velocity component.
At step 808, the first rotor velocity component may be subtracted
from the rotor velocity obtained at step 802 to determine the
modified rotor velocity. In one embodiment, the subtracting unit
212 may be configured to perform this operation. The subtracting
unit 212 may be a digital computing device or an electric hardware
device without departing from the scope of the present disclosure.
In case of a hardware device, the computing unit 210 may be
configured to output an electrical signal corresponding to the
first rotor velocity component. Similarly, the rotor unit 202 may
convert the rotor velocity into an electrical signal. These signals
(i.e., the first rotor velocity component and the rotor velocity)
may then be subtracted in the subtracting unit 212. In the case of
a digital computing device, the digital values for the rotor
velocity and the first rotor velocity component may be provided to
the subtracting unit 212 where these may be subtracted to determine
the modified rotor velocity.
Subsequently, at step 810, a first pitch angle may be generated
based on the modified rotor velocity. The subtracting unit 212 may
be configured to communicate the modified rotor velocity to the
controller 206. The controller 206, in turn, may be configured to
determine the corresponding first pitch angle. As described
previously, the controller 206 may be configured to perform this
operation by utilizing any one of a number of known technologies.
For instance, the controller 206 may include a prepopulated LUT
that includes pitch angle values corresponding to various rotor
velocities. Alternatively, the controller 206 may be configured to
store a determined threshold rotor velocity, such as a rotor
velocity that generates rated power output. The controller 206 may
subsequently compare the modified rotor velocity with the threshold
rotor velocity to generate an error signal. Furthermore, the
controller 206 may also include a LUT that stores pitch angles
corresponding to various error signals. Accordingly, the controller
206 may be configured to compare the generated error signal with
the error signals in the LUT to determine an appropriate first
pitch angle. Furthermore, in some wind turbines, the controller 206
may be configured to generate first pitch angle values for the
rotor blades 106 individually so that each rotor blade 106 may be
pitched at a different angle. In other embodiments, the controller
206 may generate one first pitch angle for all the rotor blades
106.
Following the determination of the first pitch angle, one or more
rotor blades 106 may be pitched based on a corresponding first
pitch angle, as indicated by step 812. To this end, the controller
206 may transmit the first pitch angle to the pitch actuator 214.
The pitch actuator 214 may, in turn, be configured to utilize any
known actuating means to alter the pitch angle of the blades. Some
examples of pitch actuating means may include hydraulic means,
electrical means, electronic means, and electro-mechanical
means.
FIG. 9 is a flow chart 900 illustrating another exemplary method
for reducing oscillations in a wind turbine. This method is
described with reference to FIGS. 1 and 5. Similar to the method
previously described, this method begins at step 902 by determining
the rotor velocity. Subsequently, at step 904, one or more
parameters associated with the tower 102 may be obtained. The
parameters may include tower top velocity and a second pitch angle.
In one example, the tower top velocity may be determined at step
906 and the second pitch angle may be determined at step 908. To
this end, the pitch control system 500 may include the
tower-damping unit 508. The tower-damping unit 508 may be
configured to determine the second pitch angle based on a linear
model of tower dynamics and the tower top velocity. As described
previously with reference to FIG. 5, the tower-damping unit 508 may
be configured to determine the thrust required to reduce the
oscillations and determine the second pitch angle that may aid in
generating the desired thrust.
Once the second pitch angle is computed, a modified rotor velocity
may be determined at step 910. To compute the modified rotor
velocity, it may be desirable to obtain the first and second rotor
velocity components. Accordingly, the first and second components
of rotor velocity are computed, as indicated by step 912. In one
embodiment, for this computation, the computing unit 514 may be
configured to utilize the linearized model of rotor dynamics
provided by equation (6). Using this equation, the computing unit
514 may be configured to determine a combination of the first and
second rotor velocity components ({circumflex over
(.omega.)}.sub.rc). In this model, the computing unit 514 may be
configured to employ the values of the tower top velocity and the
second pitch angle to determine the first and second components of
rotor velocity. Subsequently, at step 914, the first and second
components of rotor velocity are subtracted from the rotor velocity
obtained at step 902 to determine the modified rotor velocity. In
one embodiment, the combination of the first and second rotor
velocity components ({circumflex over (.omega.)}.sub.rc) may be
subtracted from the rotor velocity to determine the modified rotor
velocity.
Furthermore, at step 916, a first pitch angle may be generated
based on the modified rotor velocity. More particularly, the
modified rotor velocity may be communicated to the controller 206
and the controller 206 may be configured to generate the first
pitch angle. The first pitch angle and the second pitch angle may
be combined in the adder 512 to generate a combined pitch angle, as
indicated by step 918. This combined pitch angle may be transmitted
to the pitch actuator 214. At step 920, the pitch actuator 518 may
be configured to pitch the rotor blades 106 (individually or
together) to obtain a desired rotor velocity and to reduce tower
oscillations.
It will be understood that the methods of FIGS. 8 and 9 may be
repeated continuously, periodically, or at determined intervals of
time to maintain the desired rotor velocity and/or minimize tower
oscillations. In case of high turbulence or very high speeds, these
methods may not be sufficient to maintain the rotor velocity and/or
the tower oscillations within threshold limits. In such cases, the
pitch control system 116 may also be configured to power off or
shut down the wind turbine 100 until the turbulent conditions pass.
Such a measure may be taken to prevent damage to the wind turbine
100.
Furthermore, although the systems and methods described hereinabove
decouple rotor and tower dynamics to reduce fore-aft tower
oscillations and maintain effective rotor velocity, these systems
may be utilized to decouple other wind turbine dynamics as well.
For example, the decoupling unit 208 and/or 510 may be utilized in
a pitch control system to decouple rotor blade-flap and tower
fore-aft vibrations. Similarly, the decoupling unit 208 and/or 510
may be utilized in a torque controller to decouple blade-edge and
drivetrain dynamics.
In addition, the foregoing examples, demonstrations, and process
steps such as those that may be performed by the system may be
implemented by suitable code on a processor-based system, such as a
general-purpose or special-purpose computer. It should also be
noted that different implementations of the present technique may
perform some or all of the steps described herein in different
orders or substantially concurrently, that is, in parallel.
Furthermore, the functions may be implemented in a variety of
programming languages, including but not limited to C++ or Java.
Such code may be stored or adapted for storage on one or more
tangible, machine-readable media, such as on data repository chips,
local or remote hard disks, optical disks (that is, CDs or DVDs),
memory, or other media, which may be accessed by a processor-based
system to execute the stored code. Note that the tangible media may
comprise paper or another suitable medium upon which the
instructions are printed. For instance, the instructions may be
electronically captured via optical scanning of the paper or other
medium, then compiled, interpreted or otherwise processed in a
suitable manner if necessary, and then stored in a data repository
or memory.
Moreover, the various lookup tables may be incorporated in any data
repository system. For example, these lookup tables may be
implemented in a read only memory, random access memory, flash
memory, relational databases, or any other form of memory without
departing from the scope of the present disclosure. Further, these
lookup tables may be stored in a single data repository or in
individual data repositories.
Conventional rotor velocity loops typically ignore parameters such
as the tower top velocity ({dot over (X)}.sub.fa) and the pitch
angle calculated by the tower-damping loop (.theta..sub.twr) while
determining the pitch angle to control rotor velocity. Such
disregard may induce energy amplification in the rotor at tower
resonance frequencies. Sudden energy amplification may be
detrimental for the rotor, drive train, and generator. Moreover,
linear analysis reveals that the interdependence between the rotor
dynamics and the tower dynamics results in unstable rotor dynamics.
The exemplary rotor velocity loop of the pitch control system of
the present disclosure effectively reduces/eliminates the effects
of the tower dynamics on the rotor dynamics and therefore reduces
energy amplification in the rotor at tower resonance. Moreover, the
exemplary pitch control system may be employed to stabilize rotor
dynamics. Further, the fatigue loads experienced by the wind
turbines may also be reduced such that fatigue loads are within
desired working limits. For example, the systems and methods
described here may reduce tower fatigue by approximately 17%.
While only certain features of the invention have been illustrated
and described herein, many modifications and changes will occur to
those skilled in the art. It is, therefore, to be understood that
the appended claims are intended to cover all such modifications
and changes as fall within the true spirit of the present
disclosure.
* * * * *
References